1. Field of the Invention
The present invention relates to a capacitive writing device, and more particularly, to a capacitive writing device with palm rejection capability.
2. Description of the Prior Art
Referring to FIG. 1A, a conventional mutual capacitive sensor 10 is shown, which includes an insulating surface layer 11, a first conductive layer 12, a dielectric layer 13, a second conductive layer 14. The first conductive layer 12 and the second conductive layer 14 have a plurality of first conductive strips and a plurality of second conductive strips, respectively. Each of these conductive strips can be made up by a plurality of conductive pads 15 and connecting lines 19 connected to the conductive pads 15 in series.
In the process of mutual capacitive detection, one of the first conductive layer 12 and the second conductive layer 14 is driven, while the other of the first conductive layer 12 and the second conductive layer 14 is detected. For example, a driving signal is sequentially provided to each first conductive strip, and corresponding to each first conductive strip provided with the driving signal, signals from all of the second conductive strips are detected, which represent capacitive coupling signals at the intersections between the first conductive strip provided with the driving signal and all the second conductive strips. As a result, capacitive coupling signals at the intersections between all the first and second conductive strips are obtained to form an image of capacitive values.
The image of capacitive values at the time when there is no external touches is obtained as a reference. By comparing the difference between the reference image and the image of capacitive values later detected, the touch or approach of an external conductive object can be determined, and furthermore, the position touched or approached by the external conductive object can be determined.
However, the magnitude of this difference between the reference image and the image of capacitive values later detected is proportional to the area on the touch sensor approached or touched by an external conductive object, thus the area must be sufficiently large in order to be identified. Such a limitation therefore dictates the size of the pen head of a passive capacitive pen to be relatively large, preferably larger than 4 mm. The large pen head may block the view of a user from seeing the tip of the pen during writing. As a result, writing may not be accurately made at desired locations.
Moreover, the palm of the user may touch or press on the touch screen during writing, so palm rejection will be performed to ignore the locations of external objects other than that of the pen, but during writing, the pen may temporarily move away from the touch screen, causing the system to think that it should revert back to the hand-detection mode, and thus misjudging the location of the palm as the location of the pen and creating writing errors.
From the above it is clear that prior art still has shortcomings. In order to solve these problems, efforts have long been made in vain, while ordinary products and methods offering no appropriate structures and methods. Thus, there is a need in the industry for a novel technique that solves these problems.